Magnetic nanoparticles dispersed in a polymer thin film form the active recording layer in magnetic tape. To enhance the density of data that can be stored magnetically in such a composite thin film, it is desirable to use small (nanoscale) particles, at high loadings (>50% by weight), and to disperse these nanoparticles evenly within the polymer matrix. In addition, the nanoparticles must be ferromagnetic with large magnetic dipoles at room temperature to be useful for storing data. Dispersing such ferromagnetic nanoparticles evenly within the polymer matrix is problematic due to the strong interparticle magnetic forces which tend to induce aggregation and agglomeration. A method is needed to produce well-dispersed magnetic nanoparticles within a polymer matrix.